Don Wilhelm wrote:
One should not assume that any particular connector implies any kind of
standard. As an example, the 25 pin DB25 connector is used both for a real
RS232 serial port and on most PCs as a printer port.
And to make matters worse, the real RS-232 standard calls for DB-25P
(the "plug" or "male" connector) on cables and DB-25S
("female"/"socket") on equipment, on the plausible theory that if a pin
is broken it's easier to repair/replace a cable than a device.
When IBM belatedly decided to design a serial port card on the first
PCs, the engineers discovered that using DB25-Ss on PC cards was already
preempted by the printer port card people, who had already discovered
that the parallel port standard "Centroncis" connector was too wide to
fit on a standard PC card. Unfortunately they chose to use the -S
connector on the card. Since it was felt that a typical PC user couldn't
be relied upon to not plug their modem cable into their printer port if
the connectrors would fit, DB-25P was adopted as the connector on serial
port cards for PCs.
So much for "standards".
There *is* a commonly used convention for DB-9 pinouts for serial ports
on PCs...which imitates the serial card (pardon me, "asynchronous
communications adapter card") selection of the male connector. But its
more a convention than a standard. Certainly "the plug fits so it must
work" was never a safe assumption even with the original RS-232, because
there were two ends to the conversation: DCE (read "modem") and DTE
(read "terminal")...leading to the widespread need for "break-out
boxes", "modem eliminators" and "gender changers".
Guess which end the IBM card implemented?
Of course Don (who worked for IBM at that time) knows this little bit of
history, but I figure some of you young-uns might not...
73 de Maggie K3XS, who will be presenting her K2 talk at the R.F.Hill
club in Perkasie next month
--
-----/___. _)Margaret Stephanie Leber CCP, SCJP/"The art of progress /
----/(, /| /| http://voicenet.com/~maggie SCWCD/ is to preserve order/
---/ / | / | _ _ _ ` _ AOPA 925383/ amid change and to /
--/ ) / |/ |_(_(_(_/_(_/__(__(/_ K3XS / preserve change amid/
-/ (_/ ' .-/ .-/ ARRL 39280 /order."-A.N.Whitehead/
/________________(_/_(_/_______AMSAT 32844_/<[EMAIL PROTECTED]>/
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